Process for producing cobalt-aluminum bodies



United States Patent 3,429,697 PROCESS FOR PRODUCING COBALT- ALUMINUM BODIES Harvey E. Cline, Latham, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Mar. 30, 1967, Ser. No. 626,931 US. Cl. 75-170 6 Claims Int. Cl. C22c 19/00; C22f 1/10 ABSTRACT OF THE DISCLOSURE A process for producing ductile two-phase alloy bodies of cobalt-7 to 12 weight percent aluminum, is described in which the starting alloy is heated to within the range of 950 C. to 1050 C., and hot reduced in successive stages to some final size while the hot working temperature is lowered during the working stages. The final work ing temperature goes no lower than 600 C. and the final product has a fine grain size. The ductile cobalt-aluminum twophase body created is also described.

The search to locate and develop materials having high strength, good ductility and high temperature oxidation resistance has had increasing significance due to the constantly widening applications where materials having these properties are needed. One such material showing good oxidation resistance are cobalt-aluminum alloys. However, the CoAl phase which is present in such alloys is physically brittle so that various mechanical properties render the alloy unsuitable for many uses requiring strength and ductility.

It is a principal object of this invention to provide a process for producing ductile, hot-worked cobalt-aluminum bodies having controlled grain sizes delivering desirable physical properties in the alloy.

Another object of this invention is to provide a process for producing ductile hot working cobalt-aluminum bodies having high strength.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification.

Generally, the process of this invention deals with alloys made up of cobalt containing from 7 to 12 weight percent aluminum. Cast ingots of alloys within this range are heated to temperatures of from about 950 C. to 105 0 C. and then hot reduced to some preselected final size. It is important that the hot working temperature be sequentially lowered during the working stages, but in no event should the temperature be allowed to go lower than about 600 C. Subsequently, annealing can be eiiected if a grain size different from that obtained by virtue of the hot rolling operation is desired.

Considering the invention in more detail, the treatment of the cobalt-aluminum alloys with which this invention is concerned involves controlling various parameters. Specifically, the starting composition of the ingot to be worked, the initial temperature to which the ingot is heated for hot working and the final finishing temperature at the termination of the hot reducing operation are the most significant factors. I have found that to produce a ductile, oxidation resistant high temperature cobalt base material the aluminum content should range between 7 and 12 weight percent.

Hot reducing of the initial ingot can be carried out by various working means such as rolling, forging, extrusion and drawing, the particular method chosen being dependent only upon the shape of the final object desired. By heating the ingot to a temperature within the range of 950 C. to 1050 C., the material can be adequately worked without excessive grain size resulting. As the material is further reduced, the temperature during reduction should be continuously lowered so that the final grain size can be controlled. A finishing temperature of 600 C. or slightly higher will result in a material having a fine grain structure and therefore high mechanical strength. Since in alloys of these compositions the CoAl intermetallie is dispersed in stringer-like form in a matrix of Co, the lack of ductility in the CoAl is stopped from being damaging to the properties, by virtue of the ductile Co matrix.

Considering a specific example, a cobalt-10 weight percent aluminum alloy ingot was prepared by casting. This material was heated to 1000 C. for hot rolling and reduced from an initial 0.724 inch to a thickness of 0.194 inch. Then, at 800 C. the material of 0.194 inch thickness was reduced to .096 inch. Another reduction, this time at 700 C., reduced the .096 inch material to .040 inch thickness. A final reduction was carried out at 600 C. in reducing the material from 0.040 inch to a final thickness of 0.030 inch thickness. The grain size of the final material was determined to be on the order of 0.8 micron with a room temperature tensile strength of 211,- 000 p.s.i.

Several specimens were made from the material described above and then annealed at various temperatures to determine the relationship between grain size, tensile strength and relative ductility. The results of the tests conducted are shown in the following table.

TABLE I.TENSILE SAMPLE TESTS Annealing Grain size Tensile Strain Test temp. temp. mean free strength Elongation rate 0.) C.) path (p.s.i.) (percent) (minr A review of the results shown in the preceding table clearly indicates that by maintaining the grain size below about two microns in size very high tensile strengths can be obtained along with reasonably high ductility at room temperature. When the material is tested at elevated temperatures, it is extremely plastic but the tensile strength is necessarily lower at these high temperatures. The large plasticity at 1200 C. enables the alloy to be formed into intricate shapes in the same fashion as one forms plastic shapes.

It will be noted that the annealing temperatures for the anneals effected on various of the samples resulted in significant grain growth and with the grain growth comes a commensurate reduction in tensile strength. The desired structure is that of long, thin CoAl particles strung out along the tension axis. The working temperature should be reduced as the structure becomes final if high room temperature strength is desired. The grain size should be made large for high temperature creep resistance. Of course, oxidation resistance arises from the aluminum content.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in theart will readily understand. Such modifications and variations are considered to 'be within the purview and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A process for producing ductile, hot worked cobaltaluminum bodies having high strength comprising, providing a cast starting body of an alloy containing from about 7 weight percent to 12 weight percent aluminum, balance substantially all cobalt, heating the starting body to a temperature within the range of from about 950 C. to 1050" (3., hot reducing the heated starting body through a plurality of reduction stages, and gradually reducing the hot working temperature during the working stages to impart a fine grain size to the worked material, the finishing temperature being no lower than about 600 C.

2. A process as defined in claim 1 wherein the hot working temperatures and reductions are correlated to deliver an average grain size of not more than two microns in the finished product.

3. A process as defined in claim 1 wherein the composition of the alloy is 10 weight percent aluminum, balance substantially all cobalt.

4. As an article of manufacture, a hot worked body compositionally consisting of from about 7 weight percent to 12 weight percent aluminum, balance substantially all cobalt, said body having an average grain size no larger than about 2 microns.

'5. An article of manufacture as defined in claim 4 wherein said body structurally contains stringers of cobaltaluminum dispersed throughout a matrix principally of cobalt.

'6. A process as defined in claim 1 wherein plastic deformation of the cobalt-aluminum alloys is effected at temperatures no lower than about 1000" C.

References Cited UNITED STATES PATENTS 2,542,962 2/ 1951 Kinsey 75170 2,983,602 5/1961 Levinson et a1 75-170 3,211,592 10/1965 Masumoto et a1. 75-170 RICHARD O. DEAN, Primary Examiner.

US. Cl. X.R. 

